tools/virtio: use virt_xxx barriers

Fix build after API changes.

Reported-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/linux/compiler.h

@@ -0,0 +1,9 @@
+#ifndef LINUX_COMPILER_H
+#define LINUX_COMPILER_H
+
+#define WRITE_ONCE(var, val) \
+	(*((volatile typeof(val) *)(&(var))) = (val))
+
+#define READ_ONCE(var) (*((volatile typeof(val) *)(&(var))))
+
+#endif